We have investigated the effects of fluctuating carbon dioxide (CO2) concentration on water use efficiency of Zea mays L. and Phaseolus vulgaris L. We found that species-specific kinetics of stomatal movements combine with photosynthetic characteristics to influence short-term water use efficiency strongly under fluctuating environmental conditions. Specifically, under oscillating [CO2], average transpiration in Z. mays was driven higher than that observed at steady-state at the median CO2 concentration, while average photosynthesis remained fairly constant. Consequently, water use efficiencey was lower during the gluctuations in [CO2] than it was at the steady, median [CO2]. Under similar oscillations in [CO2], stomatal conductance and transiration of P. vulgaris were driven lower than observed at steady-state at the median [CO2]. A concomitant slight restriction of photosynthesis balanced this decrease in transpiration, and in this case water use efficiencey under fluctuating [CO2] remained practically constant in P. vulgaris. The frequency of oscillations in [CO2] interacted with asymmetries in stomatal opening and closing kinetics in both Z. mays and P. vulgaris to determine the extent to which average transpiration (and water use efficiency in Z. mays) departed during fluctuations from the steady-state condition at the median CO2 level.